Biological and Pharmacological Properties of Carbon Monoxide: A General Overview

Carbon monoxide (CO) is one of the most common causes of inhalation poisoning worldwide. However, it is also well known that CO is produced endogenously in the heme degradation reaction catalyzed by heme oxygenase (HO) enzymes. HO catalyzes the degradation of heme to equimolar quantities of CO, iron ions (Fe2+), and biliverdin. Three oxygen molecules (O2) and the electrons provided by NADPH-dependent cytochrome P450 reductase are used in the reaction. HO enzymes comprise three distinct isozymes: the inducible form, heme oxygenase-1 (HO-1); the constitutively expressed isozyme, heme oxygenase-2 (HO-2); and heme oxygenase-3 (HO-3), which is ubiquitously expressed but possesses low catalytic activity. According to some authors, HO-3 is rather a pseudogene originating from the HO-2 transcript, and it has only been identified in rats. Therefore, cellular HO activity is provided by two major isoforms—the inducible HO-1 and the constitutively expressed HO-2. For many years, endogenously generated CO was treated as a by-product of metabolism without any serious physiological or biochemical significance, while exogenous CO was considered only as an extremely toxic gas with lethal effects. Research in recent years has proven that endogenous and exogenous CO (which may be surprising, given public perceptions) acts not only as an agent that affects many intracellular pathways, but also as a therapeutic molecule. Hence, the modulation of the HO/CO system may be one option for a potential therapeutic strategy. Another option is the administration of CO by exogenous inhalation. As alternatives to gas administration, compounds known as CO-releasing molecules (CORMs) can be administered, since they can safely release CO in the body. The aim of this article is to provide a brief overview of the physiological and biochemical properties of CO and its therapeutic potential.